The present invention relates to a molding die and a control method thereof aimed at shortening cycle time from heating of a base material at molding to cooling of a molded product at solidifying.
In the case of the production of a large number of highly precise molded products, if a multicavity structure die is used, the size of the die increases, thus the thermal capacity increases, a long time is required for up and down of the die temperature and thus the cycle time increases, and the productivity is impaired. Further, pitch error is generated on the fixed and movable sides due to the difference of the thermal expansion in the transverse direction caused by the difference in the temperature of the die between the fixed side and the movable side, the concentricity between the center portion and the periphery of a molded product is not satisfied, and hence there are problems in quality. The difference in wall thickness between the right wall and the left wall of a cylindrical molded product 2 caused by pitch error is shown in FIG. 2(b).
A possible measure to solve the problem is to use a die of a single-cavity structure. The measure however causes another problem such as maladaptation to massive production, and another possible measure to solve the problem is to shorten the molding cycle time.
A die that can shorten the time required to heat and cool the die and thereby shorten the cycle time of molding is disclosed in JP-A-2001-009836. Further, a die that can be rapidly heated and cooled by using a heat pipe as a heat transfer medium is disclosed in JP-A-2005-138366.
In JP-A-2001-009836, a die that realizes a short molding cycle time by configuring the die so as to be able to uniformly heat and cool the whole wall of the cavity of the die for a short period of time is shown. However, since heating means is installed inside (on the wall side of) the cavity and cooling means is installed on the outer side thereof, cooling heat is transferred to the cavity wall from outside through the heating means (a tubular member, an encapsulated heat medium, and a shell part 20) at the time of cooling, thus the thermal capacity increases at the time of cooling, a large cooling heat quantity is required, and short time cooling is hardly realized. Further, in general the control of cooling is more difficult than the case of heating and the outward installation of the cooling means makes the cooling control of the cavity wall more difficult. In addition, another drawback is that a heat pipe for heating is expensive.
A die that makes it possible to rapidly heat and cool a molded product via a heat transfer column by installing the heat transfer column such as a heat pipe the front end of which comes close to the face of the die touching the molded product and further installing a heat source (for heating and cooling) capable of temperature adjustment at the rear end of the heat transfer column is shown in JP-A-2005-138366. However, since heat is transferred to the molded product via the heat transfer column, the thermal capacity corresponding to that of the heat transfer column (including the inside refrigerant in the case of a heat pipe) increases, a large quantity of heat is required at the time of heating and cooling, and the rapidness in heating and cooling of the molded product is hindered to that extent. Further, when a heat pipe is used for the heat transfer column, the speed of heat transfer increases but the drawback here is that the heat pipe is expensive.